1. Field of the Invention
This invention relates to manual control systems for aircraft and, more particularly, to a fail safe artificial force feel system for an aircraft control member generally and more particularly for a helicopter control stick.
2. Description of the Prior Art
Typically, in the manual control of aircraft embodying control surface boost systems, it is desirous to feedback artificial feel forces, such as forces proportional to the g-forces resulting from the manual control of the aircraft, to the control stick to improve the handling qualities thereof (what the pilot would feel in a non-boosted system) and reduce the possibility of over-stressing the aircraft during high speed maneuvers. Implementation of variable force feel in helicopters has been accomplished by force actuators or servo positioned springs, driven by "g" signals such as derived from pitch rate and air speed, connected to the control stick. The force actuator or force servo, is coupled to position a spring mechanism, and a servo amplifier drives the force servo to vary the force gradient of a feel spring to provide a feedback of feel forces to the control stick. A position pick-off is provided on the spring to apply inputs to the force servo, thereby commanding the servo to follow a pilot initiated stick movement in the absence of a g-force command. Such electric systems, however, are subject to failure which can drive the control stick with full authority to a hardover position, at a rapid rate. Accordingly, the prior art has resorted to extensive monitoring of the feel force servo system and/or has provided redundant feel force servo systems to, for example, continuously monitor servo system performance and thereby prevent the system from driving the stick to a hardover position. The failure detection or failure monitoring, moreover, must be very rapid, i.e., have a low threshold, which may cause unwanted "nuisance" cut offs during normal operation. In some cases, moreover, the feel system must be switched on and off to prevent undesired stick movements in response to external disturbances to the aircraft. In addition, the prior art systems reflect feedback signals to the stick such that, with the pilot's hand off the stick, the "hands off" mode, the stick responds to the signals, i.e., is moved by the signals, affecting the dynamic stability of the aircraft.
Accordingly, it is desirable to provide a fail safe force feel system which, in addition to providing the desired feedback of artificial feel forces to the stick, also prevents hardover actuation of the stick upon feel system failure, movement of the stick in the "hands off" condition and which does not require the complex electronics and hydromechanical monitoring or redundancy systems found in the prior art.